MIAMI - For a month, Cliff Lee pitched with discomfort in his left elbow. It was diagnosed as a flexor tendon injury, although not serious enough to warrant further examination. Then Lee threw 116 pitches Sunday and felt increased pain later in the game.

That is when Phillies doctors decided it was best to investigate. Michael Ciccotti, the team's head physician and leader of Major League Baseball's elbow study group, ordered an MRI exam. It revealed a mild flexor tendon strain. Lee was placed on the 15-day disabled list Tuesday night after a 6-5 Phillies win over Miami.

The Phils could be without Lee for at least three weeks.

"We want to be cautious with him, and there's no reason to try to push that," Phillies general manager Ruben Amaro Jr. said.

The unsaid fear with any elbow injury is a major operation to replace the ulnar collateral ligament, otherwise known as Tommy John surgery, which has devoured 20 major-league pitchers since the start of spring training. A.J. Burnett, a Tommy John survivor, went to bed Monday with dark thoughts.

"I said my prayers last night; I'm sure some other people did, too," Burnett said. "It wasn't great news, but it was better than the alternative."

Lee will rest for a week, at which point the Phillies will evaluate him again. Amaro noted it typically takes a pitcher twice the amount of time rested to regain strength. That makes for an optimistic timetable of three weeks.

Anything more than that could mark a devastating blow to a key component in the team's hopes for contention - or the franchise's best possible trade chip come July. Lee, either way, is central to the Phillies' future.

The soreness emerged April 17, the day after Lee threw a career-high 128 pitches against Atlanta.

"Then this last start, I started to feel it the last couple of innings, just about every throw," Lee said. "Obviously, it was pretty sore the next day, and I felt it trying to play catch. It was there on every throw."

Lee said the injury lingered for weeks but was never restricting until Sunday. He said the doctors assured him there was no ligament damage.

"I feel it [when I'm] doing random things during the day, like turning doorknobs and stuff," Lee said. "I can feel it. I have to be conscious of not doing that and letting it rest."

White Sox ace Chris Sale suffered a strained left flexor tendon April 17 and remains on the disabled list. He made his first rehabilitation start last week and could return before May ends.

Lee has no history of an elbow or shoulder injury. The 35-year-old lefthander has a 3.18 ERA this season, but his strikeout rate is down and opponents have connected for more hits than usual.

Manager Ryne Sandberg was concerned about Lee's drop in velocity during the first inning of Sunday's 8-3 victory over Cincinnati. Lee's fastball has averaged 89.7 m.p.h. this season. It was 90.7 m.p.h. in 2013 and 91.7 m.p.h. in 2012.

The Phillies do not possess vast starting depth behind Lee. Amaro said there was no immediate decision on a replacement for Lee.

Jonathan Pettibone, who made two starts in place of Cole Hamels earlier this season, has a Wednesday appointment with surgeon James Andrews in Pensacola, Fla., to examine his persistent right shoulder trouble.

David Buchanan impressed team officials in spring training and has a 3.98 at triple-A Lehigh Valley. He pitched just one inning in his last start and permitted four runs. Buchanan was removed because he reached a pitch-count limit, Amaro said. The 25-year-old righty was briefly limited before by an injury to his left (non-throwing) shoulder.

Top pitching prospect Jesse Biddle has a 3.18 ERA in 10 starts at double-A Reading.

"Everyone is going to have to come through and pitch in their role," Sandberg said.

Extra bases

Ben Revere, now healthy after suffering a stomach virus, sat for the fourth straight game. Sandberg would not commit to a regular starter in center field. "We'll see," Sandberg said. "There's some different options there. It's also an opportunity to try some different things." Tony Gwynn Jr. started Tuesday. . . . Jonathan Papelbon did not arrive at Marlins Park until shortly before the first pitch. He had to tend to a family medical issue and was excused Monday from the Phillies Phestival, which raised money for ALS research.